WASHINGTON — A Senate investigation has concluded that officials at the State Department and U.S. intelligence agencies ignored “ample warnings” of danger and failed to do enough to prevent an attack by militants on a U.S. mission and CIA base in Benghazi, Libya, in September 2012.

The bipartisan report by the Senate intelligence committee also found that no U.S. military units or aircraft were close enough to intervene or assist during the overnight attack, which left four Americans dead, including Ambassador Christopher Stevens.

“I hope this report will put to rest many of the conspiracy theories and political accusations about what happened in Benghazi,” Sen. Dianne Feinstein, a California Democrat and chair of the committee, said in a statement.

But the 85-page report, which was approved last month and declassified Wednesday, is unlikely to satisfy critics who contend that members of the Obama administration deliberately underplayed the nature of the attack because of political sensitivities during the presidential campaign.

The document recommends 18 changes intended to improve security of U.S. diplomatic and intelligence facilities overseas.

President Barack Obama has promised to “bring justice” to the leaders of the attack, but so far none of those identified as responsible have been arrested. He also has complained about the political nature of the continued criticism, which has been led largely by Republicans in Congress.

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